Anvita! When selecting any sensor you have to consider the following: time response, range, non-linearity, drift, accuracy, repeatability, among other factors. Our experience with Force Sensing Resistors (FSR) proved to be very effective for Plantar Pressures though we did have to improvise certain augmentations to the structure of the sensor and the method of sensor calibration. You may look at some of our articles for further clarifications.

typically sensors should be chosen according to their features and the environment in which it must be used. I think that the most important characteristics are:

- Type of output response: it depends on the sensor working principle and it can be a digital output (in case of Smart sensors), an active quantity (i.e., voltage or current) or a passive quantity (i.e., resistance, capacitance). According to the former you must use a proper interface;

- Sensitivity: how the sensor output response change in function of the measurans changes.;

- Resolution: the minimum measurand detectable quantity.

Other factors can be used as reference, but I think that you must start considering the former

The dynamic sensors are chosen on the basis of their frequency band and maximum load essentially. So, you should have the idea of how much high is the max signal frequence to be sensed and its max amplitude. In your case (measurements related to gait and posture) I can guess that piezoresistive transducers would be suitable.

In addition to the temporal resolution of the pressure sensor, you need to know the spatial resolution needed so the pressure of interest is measured adequately in space and your experiment will have useful outcomes. Forexample, a pressure sensor mat could give you more valuable information than single pressure point measurement. Other factors need considering include your requirement whether you need to measure in static or dynamic mode....check this for example http://www.podotech.com/diagnostics/podotech-elftman-2 this is just one supplier which may have your request off-the-shelf

We developed device, which measures the force applied by foot to the ground using sensors manufactured from PVDF film. Unfortunately polarized PVDF film has a big pyroelectric factor in the infrared range - 30×10-6 C/m2K. As a result the signal is always a sum of two big parts – piezo and pyroelectric. The pyroelectric effect must be compensated (eliminated) by measuring system.

Sample rate-, pressure area- and measuring mistake are important requirements. Adaption to the Software System (labView, or Partner Company) is important for me.

This decide about useing voltage or amperage related sensors.

One of our suppliers: [http://www.keller-druck.com/]

Greetings Ulrich

Maybe you would have to think allso about volume and possibility of placing the sensors in your setaup in order to obtain the above mentioned parameters that the producer reports.

You need to be specific about the placement of the sensors on the body for correctly interpreting gait motion. Type of output needed i.e. the change in pressure or absolute pressure - Sensors like piezo-based pressure sensors give only the instantaneous change in pressure, whereas rubber based FSRs give absolute force readings. Data acquisition interface and its speed also need to be specified.

Very interesting discussion so far, thank you all for sharing your knowledge!

If you want an affordable way to use a phone, tablet, or other Android device to talk to your force-sensitive resistors (FSRs) have a look at the Valarm Pro app and its compatible sensors: http://www.valarm.net/blog/valarm-update-yoctopuce-yocto-knob/ . You can use a dedicated phone to record all of your data and upload it to http://tools.valarm.net and receive alerts via text message and email when a sensor reading goes above or below your custom threshold. Don't hesitate to let me know if you have any questions or comments.

Cheers,

Edward.

And sometimes more than sensor, you must design a good interface, there are a lot of kind of preasure sensors, resistive, capacitive and so on. I should you to design a good sensor to ADC interface, something like this (used to temperature sensors, but if you lightly modify some details will be good for preassure too)): http://www.ti.com/lit/an/slyt173/slyt173.pdf

Also another point you must review is your ADC resolution and voltage reference, the more bit resolution of your ADC and the lower reference voltage, results in a better resolution. Good luck!

I agree with Paolo, besides the three mentioned features, it is important to consider the application, as this will define the dimensions and manufacturing materials to allow the biocompatibility and functionality needed. Moreover, piezoresistive pressure sensors are temperature dependent in comparison to the capacitive pressure sensors. And finally, resolution usually depends on the method of reading and interface ... Regards

For gait analysis you would normally use a force plate in the floor. This resolves horizontal and vertical forces vectors, most commonly using strain guage bridge circuits. Pressure measurements are usually made when studying distribution of load on the foot surface, during the gait and they do not give the same force vector information. They do show the peak loading which is somewhat related to where force vectors are directed. You can use hybrids of the two techniques with a lot more instrumentation. "Pressure" measurements usually use a full x-y array of small square force transducers (expensive). These could be things like stain guage,or capacitive based sensors. Qualitative sensors have been made using layers of polymer film, which squashes together,so that with side lighting, it can be video taped from below through a glass plate, showing high pressure contact areas. Arrays of square segments of PVDF is also only semi quantitative, as it only shows dynamic loading, with lots of artifacts. Piezo-electric arrays have been used , but like PVDF only shows dynamic loading. You might email Ed Solomon who developed a hybrid segmented force plates many years ago for gait analysis : [email protected]. Force plates tend to be expensive ,but can be built yourself, if you have modest machine shop facilities. Calibrating and setting them up, is more difficult than you might imagine, as the axes interact and signals need to be processed, so as to digitally subtract cross coupling between them, to produce X-Y-Z signals that are truly orthogonal.

Measuring actual pressure (force/sq) on a flat hard surface with a bare foot is of course very different from what really happens in a shoe say. whether you really need "pressure" ,or just the direction and peak load positions, on the foot surface during the walking cycle, is something you need to decide for your research.

Tim H

I think, you have to specify first your range of pressure measurements and accuracy needed for these measurements. Also, the expected environmental conditions and fluctuations during your pressure measurements. These info will help a lot to select your sensors.

You may use strain gauge with high resolution ...

you may use DAQ along this to record the changes

We still waiting the experimental conditions.!!

I agree with Tim Hughes but if you specify the measurement arrangement other configuration of pressure (also capacitive) could be suitable.

yes, I agree with Anthimos Georgiadis ·

You may use any strain gauge or pressure transducer connected with data acquisition and labview

I agree with Prof. Walid Tawfik, but you also have to specify what type of pressure be measured where the sensor be placed and how it be installed.

I have an example for the exprimental conditions:

Air-water two-phase flow in a horizontal pip. Two diameters of 12 mm and 25 mm.

length of the pipe about 1.5 meter. The pressure difference in the range of .5 - 20 kpa.

There are some flex sensors with Sparkfun Electronics. Just give a try if these are suitable for you. Maybe later you can think of connecting such sensors into a matrix.

Anvita! When selecting any sensor you have to consider the following: time response, range, non-linearity, drift, accuracy, repeatability, among other factors. Our experience with Force Sensing Resistors (FSR) proved to be very effective for Plantar Pressures though we did have to improvise certain augmentations to the structure of the sensor and the method of sensor calibration. You may look at some of our articles for further clarifications.

You select the one showing fastest response time and lowest hysteresis.

We have sucessfully used individual FlexiForce and Sensor Products Tactilus FreeForm sensors in fairly low pressure applications; they are also available in arrays

http://www.sensorprod.com/tactilus.php

Hi David, what the application you have used this sensor for. is it related to fluid multiphase flow, for example blood flow, ocrude oil flow , gas -liquid flow etc.

Anvita! Are you sure you need to measure pressure ? Mayby, if you are studying gait and posture analysis, you must measure force, not pressure ? Or mayby, you must measure force distribution on the surface ? This is obviously different from each other, although it is somewhat similar.

Thanks for your valuable comments.

Dr. Hamad, Per your question, we used the sensors to measure skin surface pressure during sham mammographic measurements. We have a paper describing this work in press (Academic Radiology). We have previously published work using these sensors on the head (below)

thank you for your interest,

David

Influence of probe pressure on the diffuse correlation spectroscopy blood flow signal: extra-cerebral contributions

RC Mesquita, SS Schenkel, DL Minkoff, X Lu, CG Favilla, PM Vora, DR Busch, M ...

Biomedical Optics Express 4 (7), 978-994

Thanks David, you are working in the area very close to my interest of multiphase flow of kerosene-water two-phase flow. I will try to acess the papers if the university have access to the journal.

Regards,

Faik

Anvita -- My experience is in measuring pressures in hydraulic and structural systems. In case you need pressure transducer plz keep in ,mind 1. why do I need the pressure transducer? 2. If sure then decide total range and frequency content in the phenomena u are studying.3. is it two phase flow 4. what resolution and ac curacy one demands. 5 Size to be selected. 6. Is it required to be flush with the surface where measurements are required to be made.

All such issues need be answered. Then go to catalog and select. Endevco USA gives almost very good piezo resistive transducers.

In this article we show a cheap pressure sensor  based in carbon nanostructures in a simple geometry, follow the DOI:10.1088/2053-1591/1/1/015605

Best Regards